MERS affects the lower and upper respiratory tracts, leading to coughing, shortness of breath, fever and pneumonia. The virus aims for cells in the lungs and possibly the kidneys, which may explain instances of kidney failure. But some people who have been infected report no symptoms.

Where did it come from?

Scientists say the virus may have been circulating in Arabian camels for more than 20 years. Evidence also points to bats as the initial culprit, possibly infecting camels, who then infect humans. A study published in mBio suggests that the MERS virus is capable of passing between camels and humans, but neither CDC nor the World Health Organization have confirmed the connection.

Who's at risk?

Some early victims either worked with camels, ate camel meat or drank camel milk, though it has yet to be confirmed that the virus passes from camels to humans. We don't know exactly how it spreads — perhaps through air or bodily fluids. Many patients are health care workers and family members who came in close contact with an infected person.

Who's contracted it so far, and is it here?

The first cases were diagnosed in 2012 in Jordan and Saudi Arabia. As of May 2014, there have been more than 490 MERS diagnoses (and over 140 deaths) in Saudi Arabia. Dozens more cases have been found throughout the Middle East and in seven other countries, with two in the U.S., and cases also in France, Greece, Italy, Malaysia, Tunisia and the U.K. In both the United States and the U.K, patients had been in the Middle East or were in close contact with a MERS victim.

Why is this appearing now?

Unclear. In April, officials saw a sharp uptick of MERS cases; many involved human-to-human transmission. This rise in numbers could just be because we're more aware of the illness and are better at detecting the virus.

Can it be treated? Cured?

For treatment, patients are isolated, either in a hospital or at home if the viral infection is not too severe. Those with breathing difficulties are supported with a machine that provides extra oxygen. They're given medication for fever. There's no vaccine; in the best-case scenario, it would take at least three years to develop one.

Is it lethal?

The illness is deadliest for the elderly or those with underlying conditions like diabetes and heart disease. Since 2012, about one in four patients have died. The cause can be lack of oxygen passing from the lungs into the blood.

Who should be tested?

Patients who develop a fever and symptoms of respiratory illness within 14 days of traveling from the Middle East, or while caring for a person infected with MERS.

Is there good news?

Early on, only patients with severe symptoms were diagnosed; now patients with milder cases and a better chance of recovery are being identified. So the MERS fatality rate isn't as high as researchers once thought.

Our sources: epidemiologist David Swerdlow from the Centers for Disease Control and Prevention and Andrew Mesecar, a biologist at Purdue University who researches drugs to fight viruses like MERS.